An electrostatic imaging process wherein the developer includes boron nitride

ABSTRACT

Wetting by toners of the surface of reusable photoconductors in electrophotographic processes results in image cycling in adhesion of the toner to the photoconductor surface to form a film. Such filming is reduced or eliminated by introducing into the system a small amount of boron nitride.

United States Patent nu 3,632,337

[72] Inventor Walter Crooks [56] References Cited g Calif- UNITED STATES PATENTS [21] P 3,170,790 2/1965 Clark 96/].5 [22] Filed Feb. 2, 1970 3,469,978 9/1969 Wood et al. 96/].5 [45] Patented Jan. 4, 1972 3,501,294 3/1970 Joseph 96/1 .5 [73] Asslgnee International Business Machines 3 sol 330 3 [97 C Corporation 3, ,0 0 asslers etal. 96/15 Armonk, NY. ,522, 40 7/1970 Wood et a]. 96/16 OTHER REFERENCES Deacon et aL, Proceeding Royal Society (London) A Vol. [54] ELECTROSTATIC IMAGING PROCESS 243 1958) pp. 464- 465 WIIEREIN TIIE DEVELOPER INCLUDES BORON NITRIDE jnrnary [Eammer-Gjegrg; F. Lesmes sststant xammerrammer 3 Chums No Drawings Attorneys-Hanifin and Jancin and Joseph G. Walsh [52] U.S.Cl 96/14, l17/17.5, 252/62.l,96/1 SD [51] Int. Cl ..G03g 13/14 AB$TRACT= ing y n r of the f f u ble [50] Field of Search 96/ 1.5, 1.4; photoconductors in electrophotographic processes results in 1 17/175 image cycling in adhesion of the toner to the photoconductor surface to form a film. Such filming is reduced or eliminated by introducing into the system a small amount of boron nitride.

AN ELECTROSTATIC IMAGING PROCESS WHEREIN THE DEVELOPER INCLUDES BORON NITRIDE FIELD OF THE INVENTION This invention pertains to an improvement in electrophotographic processes. By the introduction of boron nitride into the system, filming of the photoconductive surface is decreased.

PRIOR ART The use of certain additives in electrophotographic systems to reduce toner filming has been disclosed in the prior art. The addition of hydrophobic metal salts of fatty acids is disclosed in Dutch Patent Application No. 6,617,058 of Frank L. Palermiti.

BACKGROUND OF THE INVENTION Electrophotography, because of its simplicity, has during the last decade found rapid acceptance in the office copying and reproduction fields. The first electrophotographic process in its basic form is described by Chester F. Carlson in U.S. Pat. No. 2,297,691. Since Carsons discovery, many other reproduction processes based on the use of photoconductive materials have followed. Such methods have in common the use of toners, and if the photoconductive surface is to be reusable, it must be kept clean of toner accumulation. The present invention solves this problem.

In the type of electrophotography known as xerography, the essential element is the reusable electrophotoconductive insulating layer. A latent electrostatic image is formed by first charging and then exposing the electrophotoconductive insulating surface to a light pattern. The image is then developed with a thermoplastic electroscopic material, known as toner, and this material is transferred to and then fixed to paper.

The methods employed to develop images in electrophotographic printing processes are many and varied. They include cascade development as described in U.S. Pat No. 2,618,552, powder cloud development as described in U.S. Pat. No. 2,221,776, magnetic brush development as described in U.S. Pat. No. 2,874,063 and other methods including fur brush development, donor belt development, impression development and liquid spray development. The two methods most frequently employed in commercial office copying machines which make use of reusable electrophotographic insulators are the cascade mode of development and magnetic brush development. The toner particles applied in these development processes usually consist of one or more thermoplastic resin binder materials for example, polystyrene, polymethyl styrene, polymethyl methacrylate, polybutyl methacrylate, polyvinyl butyral, epoxy resin, rosin esters and the like, mixed with about 8 percent of a coloring pigment, for example carbon black, so that a colored image can be easily heat fused onto a copy sheet.

The transfer of the toner to the paper is by electrical attraction. Electrical transfer is accomplished by placing the paper in contact with the imaged area of the plate, charging the paper electrically with the same polarity as that of the latent image, and then stripping the paper from the plate. The charge applied to the paper overcomes the attraction of the latent image for the toner particles and pulls them onto the paper. Another technique for electrostatic transfer utilizes a semiconductive roll. A DC potential of the correct sign and voltage is applied between the roll and the electrode of the reusable electrophotoconductive insulating layer.

Complete transfer of toner from the surface of the reusable photoconductive insulating layer to the paper is not accomplished by these transfer methods. Accordingly, a fraction of the toner remains behind on the surface of the reusable electrophotoconductive insulating layer, and this residual toner must be removed prior to the next imaging cycle. In automatic machines cleaning of the residual toner is usually accomplished by rotating fur brush combined with a vacuum suction. The fur can be natural such as rabbits fur, or synthetic such as nylon, or Dynel, which is copolymer of vinyl chloride and acrylonitrile. Another method of cleaning residual toner from the surface of the reusable electrophotoconductive layer is web cleaning.

These methods of cleaning are efiicient in that they remove all but a small fraction of the residual toner. The frictional forces generated during the cleaning, however, promote the wetting of the surface of the photoconductive insulating layer by the thermoplastic resins of this small fraction of toner and this in turn results in a formation ofa film of toner on the surface of the reusable layer. During recycling toner-filming continues and eventually a layer of carbon and thermoplastic resin of such dimensions is present on the surface of the reusable element that copy quality is seriously impaired. This deleterious effect occurs after several hundred or several thousand cycles. After copy quality is impaired, it becomes necessary to clean or replace the electrophotoconductive insulating surface. This is inconvenient and expensive in time and material, and the solution of this problem is an object of the present invention.

SUMMARY OF THE INVENTION A method has now been discovered for preventing the thermoplastic resins of toner materials from forming films on the surface of the reusable electrophotoconductive insulating layer elements. Furthermore, this method facilitates removal of toner from the cleaning brush, thus preventing redeposition of toner on the surface of the photoconductive insulating layer.

In accordance with the present invention, a method is provided which greatly extends the life of the reusable photoconductive element and maintains the quality of its imaging characteristics. This method comprises introducing boron nitride into the system. Use of this material not only extends the life of the reusable photoconductive element but also greatly extends cleaning brush life, particularly when the brush fibers are manufactured from a copolymer of vinyl chloride and acrylonitrile.

According to the present invention, there is incorporated into the system a small amount of boron nitride, a compound normally in the form of a slippery white solid with a hexagonal layer structure. The hexagonal units have alternate B and N atoms 1.45 A. apart with angles of The distance between the layers is 3.34 A. At extremely high temperature and pressure the hexagonal layer structure is converted to a cubic form with a diamond-lie structure. The cubic form is not a lubricant and not suitable for use in the present invention. For the present invention, the boron nitride must be in the hexagonal layer form and it is only this usual form of boron nitride which is referred to in this application by the term boron nitride lubricant.

There are a variety of ways of introducing boron nitride into the copying system such that it eventually becomes transferred to the photoconductor surface and thereby inhibits toner filming. Furthermore, when conventional brush cleaning of the photoconductor surface is performed, then the boron nitride also transfers to the brush fibers and in so doing prolongs brush life.

Although the real mechanism has not been conclusively elucidated, it is considered the boron nitride is attracted to the surface of the photoconductor where it forms a film. This extremely thin film of boron nitride prevents the thermoplastic resins of the toner from filming on the surface of the reusable element. A similar phenomenon occurs on the brush fibers in conventional brush cleaning and the boron nitride extend the life of the fibers by reducing wear without subtracting from the brush cleaning ability.

Some of the practical ways of introducing boron nitride into the system are listed below. The surface of the photoconductor can be treated with boron nitride prior to cycling. When brush cleaning is used the cleaning station housing can be coated with boron nitride and the cleaning brush can be designed to contact the housing. The cleaning brush flicker bar (knockoff bar) can be coated with boron nitride, or boron nitride can be extruded from the knockoff bar at a metered rate and hence the brush becomes coated with it and it is transferred to the photoconductor surface. A web can be impregnated with boron nitride and can be brought into contact with the photoconductive insulator surface intermittently or continually. The photoconductive insulator can be dry sprayed or solvent sprayed with the boron nitride prior to the cleaning brush, preferentially, but not necessarily, restricted to this position, intermittently or continuously. The paper to which the toner is to be transferred, can be dry sprayed or solvent sprayed with the boron nitride prior to the transfer process. The housing of the developer section, or any other part of the developer section, or any other part of the developer section which is contacted by the toner, or the toner and the carrier, can be coated with the boron nitride. The carrier surface can be treated with boron nitride prior to adding to the toner to form the developer mix. The toner-carrier developer mix can be continuously or intermittently dry sprayed or solvent sprayed with boron nitride. The boron nitride can be metered into the developer system at a slow rate of addition sufficient to ensure that the surface of the photoconductive insulator does not become toner-filmed. When donor belt development is employed, the donor belt can be continuously or intermittently dry sprayed or solvent sprayed with boron nitride. A preferred mode of introducing the boron nitride into the system is to include it in the tonercarrier developer mix. When this method is used, the particle size of the boron nitride must be small, preferably less than 5 microns. Most toners have an average particle size in the 5-20p.range. In the cascade mode, carrier beads are in the 200-l ,OOOu range. The most efficient boron nitride which has been used so far has an average particle size less than 5 microns. Thus, it may be said in summary that the boron nitride must be present as finely divided particles which have a particle size lower than that of the toner, in those cases where it is added to the toner-carrier mix. When boron nitride is added to the toner mix, only a small amount is needed. As little as 0.1 percent is effective. There is no advantage in going over percent. All these methods of introducing boron nitride into the system are simply ways of carrying out the process of the present invention, i.e. maintaining a thin deposit of boron nitride on the surface of the photoconductive plate.

The general nature of the invention having been set forth, the following examples are now presented as to the specific operation of the invention. The specific details presented are for purposes of illustration and not limitation. Parts and percentages are by weight unless otherwise indicated.

EXAMPLE I Boron nitride of less than S-micron particle size was added to Hunt Graph-O-Print toner at a 0. 1 percent level of addition on a weight basis. (Hunt Graph-O-Print toner is a pigmented polystyrene-nbutyl methacrylate copolymer manufactured by Philip A. Hunt Chemical Co.) After mixing, the toner and additives were cycled with, an organic photoconductive insulating film containing 8 percent by weight of a polyester adhesive known as 49000 Mylar adhesive, a 60/40 ethylene terephthalate/isophthalate copolymer from DuPont. The organic electrophotoconductive insulating film comprised a one-to-one molar ratio of polymerized vinylcarbazole and 2,4,7-trinitro- 9-fluorenone disclosed and claimed in an application of Shattuck and Vahtra, Ser. No. 556,982, filed June 13, 1966, now US. Pat. No. 3,484,237. The cycling was carried out on a laboratory toner-cycling machine which simulated copying machine cleaning conditions. The toner and additive mixture was continually rubbed against the organic electrophotoconductive insulating film with rabbits fur, a material used in the cleaning brushes for electrophotoconductive insulating films. After 10,000 cycles the film was examined with a microscope to detemiine the degree of filming which had occurred. It was next compared with two controls, one of which had been cycled for the same number of cycles with toner without the boron nitride a nd rabbit's fur, and the other for the same number of cycles with rabbits fur alone. The system showed slight filming compared to rabbits fur alone. The other control was filmed heavily.

EXAMPLE II Example I was repeated, with the level of addition of the ad ditive increased to 0.25 percent on a weight basis. All other conditions and operations were as in example I. In this case the test system was not filmed to any degree and compared equally with rabbits fur alone. The other control was filmed heavily.

EXAMPLE III This experiment was performed on a drum robot copying machine which simulated exactly the cycling conditions of a xerographic copying machine. In one cycle the organic electrophotoconductive insulator, which is the same as that described in example I, was charged, exposed, cascade developed, corona transferred, corona precleaned and cleaned, all at 4 inches/sec. The developer mix contained 2 percent Hunt Graph-O-Print toner, of which 5 parts in parts was boron nitride powder, and 98 percent Exon-Orasol red coated glass beads. A dynel brush was used in the cleaning station, and engagement and speed of rotation were set at optimum conditions for a developer mix containing no filming inhibitor. The degree of filming was measured as a function of loss of gloss of the surface of the reusable element. It was found that the average initial gloss reading was 146 and after 500 cycles was 133 as measured on a Photovolt-Gloss-Meter. It remained at this value thereafter until the experiment was terminated at 5000 cycles. Without the presence of boron nitride, the initial gloss was and was 120 after only 500 cycles and was 80 after 2,000 cycles. In the former case, print quality was excellent and no toner filming was evident. In the latter case, the surface of the organic electrophotoconductive insulator was visibly heavily filmed with toner. Print quality was adversely affected.

EXAMPLE IV In a further experiment which was performed on a drum robot copying machine using a cascade mode of development with boron nitride contained in Hunt Graph-O-Print, boron nitride filming of the photoconductor was allowed to occur to the point where developed image quality was adversely affected. At this point Celite -8, :1 small particle size (1-6;; range) diatomaceous silica, mined by Johns Manville, Celite Division, was added to the developer at a 2 percent level of addition by weight based on toner. It was discovered that the developed image quality returned to the high level evident at the beginning of the experiment indicating the boron nitride film to have been eroded from the photoconductor surface.

EXAMPLE V This example of example III was again performed on a drum robot copying machine which simulated exactly the cycling conditions of a xerographic copying machine. In one cycle the organic electrophotoconductive insulator, which is the same as that described in example I, was charged, exposed, magnetic brush developed, corona transferred, corona precleaned and cleaned, all at 4 inches/sec. The developer mix contained 2 percent Hunt Graph-O-Print toner, of which 5 parts in 100 parts was boron nitride powder, and 98 percent polytetrafluoroethylene grit carrier. ln magnetic brush development it is essential that the boron nitride is uniformly dispersed in the toner and that no boron nitride agglomerates are present otherwise developed image quality is adversely affected. To ensure dispersion, the boron nitride was added to the toner in a Waring Blender with care. When the blade in the Blender is allowed to turn continuously at a speed sufficient to provide dispersion, heat buildup occurs within 30 seconds and toner fusion occurs. However, using an on-off procedure, dispersion is accomplished in two minutes and no heat buildup occurs. In this experiment, cleaning conditions were set at optimum conditions for a developer mix containing no filming inhibitor. The degree of filming was again measured as a function of loss of gloss of the surface of the reusable element. It was found that the average initial gloss reading was 146 and after 5,000 cycles was 140 as measured on a Photovolt-Gloss-Meter at which point the experiment was terminated. In a comparable experiment without boron nitride, gloss had decreased to 122 after 2,000 cycles on a Photovolt- Gloss-Meter and the photoconductor was visibly filmed.

While the invention has been shown and described with reference to preferred embodiments thereof, it will be appreciated by those skilled in the art that many variations in form may be made therein without departing from the spirit or scope of the invention.

What is claimed is:

1. In an electrophotographic process wherein a latent electrostatic image is formed on a photoconductive plate, developing material is applied to said image and the developed image is transferred to a receiving sheet, the improvement which comprises applying to the surface of said plate a thin film of boron nitride in the hexagonal layer form.

2. A process claimed in claim 1 wherein the boron nitride is applied as particles less than 5 microns in size mixed with the developer material.

3. A process as claimed in claim 1 wherein boron nitride is applied to a cleaning brush which is in contact with the surface of the photo conductive plate. 

2. A process claimed in claim 1 wherein the boron nitride is applied as particles less than 5 microns in size mixed with the developer material.
 3. A process as claimed in claim 1 wherein boron nitride is applied to a cleaning brush which is in contact with the surface of the photo conductive plate. 